Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 132134
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
334 PM MDT Wed Sep 13 2017

.SHORT TERM...(This evening through Friday night)
Issued at 330 PM MDT Wed Sep 13 2017

Current KCYS radar loop shows scattered showers with a few
thunderstorms across southeast wyoming and the northern Nebraska
panhandle. Activity is also initiating just south of the southern
Nebraska panhandle and should move into the I-80 corridor through
the evening. The main impact from these thunderstorms will be
gusty outflow winds and the potential of some strong gusts as
well. Winds have been breezy at times mainly across southeast
Wyoming, although some of these winds may be due to convective
activity in the area as winds have not been all that persistent
through the afternoon. Expect showers and thunderstorms to linger
through sunset, and then diminish around midnight. However, some
rain shower activity may move back into Carbon county as the
upper level trough near the Calif coastline begins to eject
northeastward.

For Thursday, the beginning of a more unsettled and cooler weather
pattern will begin as the Pacific upper level trough moves
towards the area. Meanwhile, a stronger Pacific system will dig
southward across the Pacific northwest associated with the Polar
jet steam. The upper level ridge, which has led to the recent
record/near record highs, will continue to weaken and translate
eastward into the plains as the flow aloft becomes southwesterly.
Expect a higher coverage of showers and thunderstorms for
Thursday as the first of two cold fronts makes its way through
southeast Wyoming, so increased POP between 30 to 60 percent over
most locations with values below 25 percent for the lower valleys
of Carbon county.  Activity will most likely come in two rounds, one
with the heating of the day and a second one Thursday night as
the upper level trough in Calif moves overhead. Temperatures on
Thursday will begin to lower in response to the Pacific upper
level trough, with highs likely in the 70`s west of I-25 and low
to mid 80`s east of I-25 with temperatures lowering through the
afternoon.

Models show the second cold front and stronger upper level trough
from the Pac NW impacting the forecast area on Friday and Friday
night. Expect a brief break in the precip sometime late in the
morning through the afternoon hours as an area of subsidence aloft
moves along the I-80 corridor. Another round of showers and
thunderstorms along with cooler air will then move into the
region with highs likely in the 60`s along and northwest of a
Laramie to Lusk line. It will likely be cold enough for snow
above elevations of 10k feet in the Snowy and Sierra Madre ranges
Friday night as 700mb temperatures drop below 0-2c. Outside of the
mountains, there is a good chance of rain showers and a few
evening thunderstorms, especially north of I-80. Kept likely POP
in the forecast with values between 25 to 40 percent along the
I-80 corridor through early Saturday.

.LONG TERM...Saturday through Tuesday...

The models are in rather good agreement at showing the main upper
trough swinging northeastward from Wyoming into the northern Plains
on Saturday.  Highs on Saturday will be cool, around 10-15 degrees
below normal.  While the best chance of precip will be mostly Friday
night, will keep slight chance PoPs for much of the CWA through
Saturday morning.  Sunday and Monday look like rather pleasant days
as heights aloft rise and temps moderate back toward normal.  Both
the GFS/Euro show an area of precip in isentropic lift developing by
Sunday aftn/evening over northeast Colorado that may impact portions
of the southern Nebraska Panhandle.  A deep upper trough is progged
to dig into the Pacific Northwest and Great Basin by early next
week.  Other than breezy southwest winds developing for areas to the
west of the Laramie Range, should not see much impact from this
system through at least early in the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1206 PM MDT Wed Sep 13 2017

VFR conditions through the taf period. Isolated storms will be
possible this afternoon over much of the area. Gusty and erratic
outflow winds will be the main aviation hazard. West to northwest
winds will gust to 20-25 kts through 00Z at the southeast WY
sites.

&&

.FIRE WEATHER...
Issued at 237 PM MDT Wed Sep 13 2017

Will continue to monitor observations this afternoon across zones
301, 303, and 304 for elevated Fire Weather conditions. At this
time, winds have not been persistent enough while relative
humidities have fluctuated between 15 to 25 percent, so do not
anticipate Red Flag conditions at this time. In fact, humidities
have been higher than expected west of I-25 with values between 20
to 35 percent. Concerns will trend lower through the weekend as a
series of Pacific systems move across the region resulting in
cooler temperatures, higher humidities, and a good chance of
rainfall.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...TJT
LONG TERM...ZF
AVIATION...ZF
FIRE WEATHER...TJT



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